The control of ordinary fuel vapor emissions from vehicle fuel tanks is handled adequately by conventional vapor storage and retrieval systems. No particular effort is made to reduce the level of vaporization that normally occurs within the tank. However, the prospect of more strict regulations requiring that the vapors displaced from a tank when it is filled also be recovered has raised concerns about the adequacy of storage canister capacity. Consequently, there has been increased interest in finding ways to reduce the level of fuel vaporization within the tank in the first instance. One of the primary causes of increased fuel vaporization is fuel heating.
Fuel heating and vaporization may occur just from hot outside temperatures as the vehicle sits, generally referred to as diurnal losses. Another cause of fuel heating, especially in vehicles with fuel injection systems, is the return of unburned fuel to the tank. More fuel is pumped through the fuel rails than is burned, and is inevitably warmed somewhat. Warmer returned fuel, when mixed with the stored fuel, raises the overall fuel temperature The more thorough the mixing, the more thorough the heating and the greater the consequent fuel vaporization, generally called running losses Many vehicle fu.RTM.l systems place the pump inside the tank, with the pump inlet near the bottom of the tank. In order to assure a constant supply of fuel at the pump inlet to avoid pump starvation during cornering, a reservoir may be built around the pump. Some systems go farther than that, and use the force of the returned fuel stream to power a venturi nozzle and force more stored fuel toward the pump inlet. More fuel may be forced toward the pump than the pump needs, causing the reservoir to overflow. This enhances the mixing of returned and stored fuels, increasing the heating effect and consequent running losses.